1. Field of the Invention
The present invention relates to a heating apparatus which is capable of sensing, by means of a vapor sensor, the state of gas or vapor generated from a heated substance in accordance with the state of heating, so as to automatically determine the timing of completion of heating of the substance, thereby optimizing the heating operation.
2. Description of the Related Art
A known heating apparatus for heating a material in a heating chamber has a sensor capable of sensing a change in the state of vapor generated from the heated material. In this known heating apparatus, air is introduced from the heating chamber and is then returned into the heating chamber through a return air passage. The sensor is disposed in this return air passage.
This type of heating apparatus is disclosed, for example, in Japanese Patent Unexamined Publication Nos 59-191813 and 58-127017. In the apparatus disclosed in these publications, a sensor is provided, rather than an exhaust passage for ventilating the heating chamber, in a return passage through which air that has been extracted from the heating chamber through an extracting passage is returned to the heating chamber. According to this arrangement, the vapor generated from the heated material is sensed substantially in the same heated state as that of the heated material without being cooled. This arrangement, however, has a drawback in that sensing errors may occur.
Namely, if the position of the opening of the return passage opening to the heating chamber is not precisely determined in relation to the opening for introducing air from the heating chamber to the outside, the vapor generated by the heated material is undesirably mixed with chilled air from an air supply opening before the vapor is introduced into the heating chamber from the return passage, resulting in that the temperature of the vapor is lowered to impede the automatic control of the heating operation.
Problems are encountered even when the opening of the return passage is precisely located. The vapor is recycled between the heating chamber and the return passage. In the beginning period of the vapor generation, the sensing of the vapor by the sensor is conducted relatively easily because the concentration of the vapor is increased. However, when the quantity of the vapor is decreased due to the stopping of heating or when the heating has been suspended to prepare for the next heating cycle, detections of the change in the heating state tend to be delayed due to stagnation of the vapor.